The present invention relates to increasing the molecular weight during a thermal treatment of polyester in combination with a latent heat granulation. With the newly-developed method, an SSP (Solid State Postcondensation) can be directly combined with an underwater granulation. The method differs from a conventional solid state postcondensation by an increase in the molecular weight being possible without additional heat input and hence only by using the residual heat and the crystallisation heat present. A characterising element is improved water separation and dehumidification during the granulation. Only in this way is an increase in viscosity possible even with a small granulate of an average particle weight less than 20 mg.
To date, it has been typical of solid state postcondensation plants for polyester that, after the water separation of the granulation, once again drying and crystallisation with heat supply is provided in order to achieve the required reaction temperature and to prevent degradation by hydrolysis.
Polyester is generally produced under vacuum in a melt phase at 275 to 300° C. final temperature. The product is used directly as melt or processed to form granulate and thus is made available for further processing via a melting extruder. According to the application, different quality demands are made, above all with respect to the intrinsic viscosity.
For further processing to form bottles, films or industrial yarns, higher strengths are required than in the synthetic fibre industry. The additionally desired viscosity increase can be effected in an extended polycondensation, relative to that which is normal in the case of fibre raw materials, in the melt phase itself or in a solid state postcondensation. In the case of solid state postcondensation, a granulate is dried again and heated and then polycondensed in the vacuum or in a gas flow at temperatures of 200 to 225° C. Vacuum or a gas flow are required for progress of the reaction in order to discharge the resulting by-products (ethylene glycol, water and also other highly volatile, organic materials).
In U.S. Pat. No. 4,064,112, the thermal damage which arose earlier during the melt condensation at an intrinsic viscosity of more than 0.7 is described. The nowadays common method of solid state postcondensation in an inert gas flow in the shaft reactor at approx. 220° C. is described. It is likewise disclosed how the residual water from the granulation or water absorbed by the polyester due to hygroscopy must be removed. For this purpose, a drying unit is installed before the solid state postcondensation. Furthermore, it is described how the agglutination arising during heating must be avoided by movement during the crystallisation. U.S. Pat. No. 4,064,112 also relates to the removal of highly volatile by-products in a solid state postcondensation (dealdehydisation).
The latent heat crystallisation is described in EP 1 608 696. In this method, the granulate is only cooled to such an extent that, after removal of the cooling water with an agitating centrifuge, the inherent heat remaining in the particle can be used directly for crystallisation. The aim is to avoid agglutination by means of a vibrating channel downstream of the agitating centrifuge. The method serves for the purpose of achieving a sought degree of crystallisation for the further processing. It was shown later that no agglutination occurs even without an agitating bed (cf. DE 103 49 016 with the subsequent DE 10 2006 013 062).
In a further step for process intensivisation, a method is represented in WO 2006/06030 in which high intrinsic viscosity is achieved in a melt polycondensation, a low content of acetaldehyde being achieved by skilful process management. Hence, for the first time bottle granulate was able to be produced on an industrial scale without solid state postcondensation. For this new so-called melt-to-resin process (MTR®), latent heat granulation was used. In addition, the inherent heat remaining in the granulate and the generated crystallisation heat was used directly for reducing the acetaldehyde content by means of a thermal treatment with air at 175° C. An increase in viscosity was not sought after.
In U.S. Pat. No. 7,674,878, a latent heat granulating method, in which a non-adhering granulate is made available for further processing at a controlled temperature level by shock cooling, is described.
A reduction in viscosity due to moisture after the agitating centrifuge must be avoided in the above-described MTR® method. The result of a further development is emphasised in WO 2009/027 064. An improved agitating centrifuge which is distinguished by a tangential entry of the granulate-water mixture into the agitating centrifuge is presented therein. The diameter of the agitating centrifuge is widened at the top. After the main dewatering in the lower part, the residual water is centrifuged off with an increased diameter. The vapour withdrawal is assisted by centrally fed air. Dry air is also conducted in counterflow at the granulate outlet and the following silos in order to avoid entrainment of moisture in the thermal further processing (dealdehydisation). A reduction in viscosity due to hydrolysis can thus be extensively avoided.
In U.S. Pat. No. 5,292,865, the essential elements of a method with melt polycondensation and dealdehydisation are described. The treatment with dry air and a thereby arising increase in viscosity at 170 to 185° C. with a treatment time of 10 to 12 hours are emphasised. In addition, a latent heat crystallisation method is described in which the problematic agglutination with other crystallisation methods does not occur. The representation of a temperature-controlled latent heat crystallisation which permits an optimal operating window for the dealdehydisation even for a small particle and which is crucial for industrial use is however not described in this patent. The granulate must merely be dried in a complex manner according to this method in order that an increase in viscosity takes place, which outweighs the reduction in viscosity due to hydrolysis.
However, various economic and qualitative disadvantages must be ascribed to the established methods.
1. Thermal Stress in the Melt Postcondensation
At a higher intrinsic viscosity, the melt viscosity also increases in the polycondensation reactors. The melt flow and above all the surface formation and hence the discharge of the by-products are made difficult. As a result, higher temperatures or long dwell times must be provided. Hence, degradation reactions which run contrary to the increase in viscosity and lead to material losses are promoted.
2. Energy and Investment Expenditure in the SSP
The pre-product in the form of a granulate must be dried and heated. The product is thereby crystallised and complex, mechanically moving intermediate steps must be provided in order to prevent the agglutination of the granulate particles triggered during the crystallisation. The treatment in fluidised beds leads to dust formation and demands corresponding filter plants for a stable operation.
3. Thermooxidative Damage to the Granulate During an SSP
According to conventional experience, the solid state postcondensation in a gas flow starts detectably only from approx. 180° C., in which, when using air, already the beginning of oxidative damage must however be taken into account. At lower temperatures (approx. 160° C.), a solid condensation could also be implemented with air with correspondingly long reaction times. However, in order to avoid large apparatus via an acceleration of the reaction, essentially higher temperatures must in practice be operated at, an inert gas then requiring to be used in any event: from approx. 190° C., the oxygen component of the air leads to significantly visible quality losses; on the other hand, inert gases are expensive and must be recirculated. For this purpose, oxygen and any by-products occurring must be removed via complex purification plants.
4. Reduction in Molecular Weight Due to Hydrolysis
Polyester is hygroscopic and absorbs moisture during the granulation or during storage in an air atmosphere. Before further processing, the material must therefore be dried, generally in a solid bed or a fluidised bed with air or an inert gas. The residual moisture in the granulate, but also the moisture in the drying air or in an inert gas circulation, leads to hydrolysis and hence to a reduction in viscosity.
5. Hydrolysis During Reuse of Recycled Pet Bottles in New Drink Bottles
After use, PET bottles are collected, sorted, washed and ground into flakes. For reuse, residual contamination must be expelled. For this purpose, the material is treated thermally under vacuum or inert gases. Residual moisture leads to hydrolysis and hence a reduction in viscosity. For reuse of this material in new bottles, this viscosity must be increased again.
It is problematic in particular in the above-mentioned production method of polyester pellets, in particular in the already known latent heat crystallisation method, that, in the case of the method implementations known from the state of the art, during crystallisation of the pellets a reduction in intrinsic viscosity and hence in molecular weight of the polyester used has always been able to be observed up to a certain degree. In the case where such a reduction in molecular weight of the polyester granulate should be counteracted, a further subsequent solid state postcondensation was therefore always absolutely necessary.